Nezumi's mechanical Mice inhabit the city. They are devices that Nezumi created to monitor or watch what is happening in No.6 city. With their appearance they can easily send messages or do different tasks. In the original novel Nezumi's first letter tells Karan that "Shion is safe, worry not. Escaped to West Block. Any replies to this mouse. Brown (Cravat) brings news of safety, black (Moonlit) brings news of change or abnormal occurrence,"
They take on the appearance of normal mice with slightly reddish eyes.